1. Field of the Invention
This invention generally relates to an ion source for a mass spectrometer, and more specifically to an ion transfer tube for transporting ions between regions of different pressure in a mass spectrometer.
2. Description of Related Art
Ion transfer tubes, also referred to as capillaries, are well-known in the mass spectrometry art for transporting ions from a spray chamber, which typically operates at or near atmospheric pressure, to a region of reduced pressure. Generally described, an ion transfer tube typically consists of a narrow elongated conduit having an inlet end open to the spray chamber, and an outlet end open to the reduced-pressure region. Ions formed in the spray chamber (e.g., via an electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) process), together with partially desolvated droplets and background gas, enter the inlet end of the ion transfer tube, traverse its length under the influence of the pressure gradient, and exit the outlet end as a supersonic expansion. The ions subsequently pass through an aperture in a skimmer cone through regions of successively lower pressures and are thereafter delivered to a mass analyzer for acquisition of a mass spectrum. The ion transfer tube may be heated to evaporate residual solvent (thereby improving ion production) and to dissociate solvent-analyte adducts.
The number of ions delivered to the mass analyzer (as measured by peak intensities or total ion count) is partially governed by the flow rate through the ion transfer tube. It is generally desirable to provide relatively high flow rates through the ion transfer tube so as to deliver greater numbers of ions to the mass analyzer and achieve high instrument sensitivity. The flow rate through the ion transfer tube may be increased by enlarging the tube bore (inner diameter). However, increasing the cross-sectional area through which the ions and gas are transported has a detrimental effect on the efficiency of heat transfer to the ion/gas flow. Enlargement of the ion transfer tube beyond a certain point achieves no further gains in sensitivity, because the benefit produced by increased flow rate is offset by significantly reduced desolvation/adduct dissociation rates. Of course, the heat transfer to the ion/gas flow may be increased by raising the tube wall temperature, but the maximum temperature at which the tube may be operated will be limited by material considerations, as well as the tendency of certain analyte molecules to undergo thermal dissociation.
U.S. Pat. Nos. 6,583,408 and 6,803,565 by Smith et al. disclose a mass spectrometer having a parallel arrangement of multiple heated capillaries for transporting ions from an ESI spray chamber to an ion funnel. The multiple capillary configuration enables both high flow rates and good heat transfer efficiencies. However, the ion/gas flows emerge from the exit ends of the capillaries as a geometrically complex set of multiple expansions, which (although suitable for use with the ion funnel) could not be easily interfaced to a conventional skimmer structure having a single aperture.
U.S. Pat. Application No. 2006/0186329 by Gebhardt et al. discloses an ion inlet of an ion source for a mass spectrometer having a multichannel plate that functions similar to the multiple heated capillaries of the Smith patents described above. That is, the multiple channels in the multichannel plate receive and guide ions and background gas and provide a large area entrance from the source into an ion funnel. Also in this case, the multichannel plate could not be easily interfaced with a conventional skimmer structure having a single aperture.
Another consideration is that with increased wall surface area in a multiple capillary or multichannel arrangement, more ions will be lost due to discharge when they come into contact with the wall surface area.
In view of the foregoing discussion, there is a need for an ion transfer tube that enables high flow rates while maintaining good heat transfer efficiency, and is capable of being interfaced to a conventional skimmer or similar structure.